Stability of Low-index Surfaces of Cs2SnI6 Studied by First-principles Calculations

Cs 2 SnI 6 is a stable and environmentally friendly halide perovskite material with great potential for photovoltaic and optoelectronic applications.While the surface properties are of paramount importance for device fabrications, there have been no such theoretical studies on this material.Using density functional theory calculations with the SCAN+rVV10 functional, the (001), ( 011) and ( 111) surfaces of Cs 2 SnI 6 were studied to reveal their thermodynamic stability.We constructed seven models for these surfaces, including two along the (001) orientation (CsI 2 -and SnI 4 -terminated surfaces), two along the (011) orientation (I 4 -and Cs 2 SnI 2 -terminated surfaces) and three along the ( 111) orientation (non-stoichiometric CsI 3 -, Sn-and stoichiometric CsI 3 -terminated surfaces).Because most of the surfaces are non-stoichiometric, their relative stability depends on the experimental preparation condition, which is reflected by the chemical potentials of the constituent elements in the calculation.By determining the allowed chemical potential region, the thermodynamic stability of these Cs 2 SnI 6 surfaces is analyzed.The results show that the surface energies of the ( 001) and ( 011) surfaces are affected by the chemical potentials, while the stoichiometric CsI 3 -terminated (111) surface is unaffected by the chemical potentials and is energetically the most stable surface of Cs 2 SnI 6 .Thus, the observed exposure of (111) surface of Cs 2 SnI 6 crystals in several recent experiments is determined to be driven by thermodynamics.